The RB2 Standing Valves are complete units that sit on the Bottom No- Go Shoulders of the R Nipple. This no-go shoulder restricts the downward movement, but these valves are not locked into the R Nipple and do not restrict upward movement. They are used to hold a column of fluid in the tubing if the well stops producing which can make swabbing operations easier.
The RB2 is generally placed in the tubing string when there is a need to retain fluid from being reabsorbed back into the wellbore formation.
A good example of this would be to place the standing valve above a submersible pump.